The present invention relates to retroreflective articles having prismatic retroreflective elements.
Micro Electro Mechanical Systems (MEMS) comprises of a range of technologies that enables the fabrication of miniature devices such as sensors and actuators. “Optical MEMS”, or “Opto-MEMS”, has recently been replaced with the term “MOEMS”, for Micro Opto Electro Mechanical Systems. MOEMS derive their functionality from the miniaturization of Optics, Electronics and Mechanics. They employ MEMS to sense, detect or manipulate data, to change light intensity or phase modulation by using refraction, reflection or diffraction principles.
Corner Cube Retro-reflector (CCR) devices are known in the art to provide a variety of applications ranging from reflecting beams coming from the head lights in road signs to miniature and efficient transmitters in a free-space optical communication systems, or remote sensing instruments.
As is known in the prior art, a CCR consists of three mutually orthogonal flat mirror surfaces, forming a concave corner as shown with reference to FIG. 1. A ray of light entering the CCR is reflected back parallel to the incident light, if the light comes from a quadrant of a hemisphere defined by the concave side of the CCR and hits the CCR within a particular area defined by the incident direction. It acts similar to a flat mirror reflecting a normal light ray to its source. While a flat mirror may reflect light from a hemisphere of directions, a CCR is only responsive to light coming from a quadrant of a hemisphere defined by the concave side of the CCR. The advantage of using a CCR over flat mirrors is that the light ray does not need to hit normal to the plane of the mirror to retroreflect.
As illustrated with reference to FIG. 2, by tilting and realigning a mirror or mirrors of the CCR, light can be intermittently reflected away from the direction of the interrogating light source, thereby transmitting a digital signal.
Passive transmitters are known in the art that utilize a micromachined CCR, which can operate at data rates up to 10 kbps over ranges of over 150 meters while only consuming 1 mW of power. The appealing quality of the CCR is that it is extremely low power because it dose not require a dedicated light source. The cost of transmission is limited to the energy required to deflect one of the mirrors, which in the case of a MEMS CCR is very small. The CCR transmitters are a promising technique for ultra-low power, high endurance applications. Using the CCRs as wireless communication links has several advantages including low power, small size, and low cost. The transmitter with a CCR may consume minimal power since it transmits data by reflecting external power. A corner cube retroreflector removes the need for precise angular alignment between the laser source and the CCR; all that is needed is to ensure that incident light is in the acceptance angle quadrant of the CCR.
Batch fabrication technology for micromachining can also yield low cost systems. As a result, remote stations with micromachined sensors and a CCR transmitter have the potential to be low powered, autonomous, small and inconspicuous, so that a large distribution of remote stations may be possible.
Over the years the CCRs have gained importance in many fields. The main advantages in the micromachined CCRs lies in the ability to batch fabricate them. For instance, sensors as employed in combat applications may use the CCRs to transmit information regarding the environmental conditions to an interrogating aircraft with a laser. Additionally, development work has been done using the CCR for wireless digital communications. The CCRs can be used in vehicle-to-vehicle optical two way communication. They are also used in making X-ray image intensifier screens, reflective liquid crystal displays, etc.
The literature contains several techniques to fabricate a CCR using micromachining and other tools. These techniques include “popping-up” of mirrors to make 3-D structures, bonding of mirrors, epitaxial growth of silicon crystal and direct formation of a CCR using cutting tools. The major drawbacks in these methods were the size of the CCR, acceptance angle of the packaged CCR, the abundance of unused chip area and tedious assembly due to the mechanics of the fabrication process.
MEMS technology has been used to fabricate the electrostatically modulating CCRs. A MEMS modulating CCR consists of two fixed mirrors, and one movable mirror. This modulation of one mirror helps in wireless digital communication. Surface micromachining technology allows fabrication of mirrors parallel to the plane of the wafer. The 3-D structures are then achieved by popping one mirror surface out of the plane of the wafer. Different kinds of popping mechanisms have been tried by researchers. Micro manipulators, ultrasonic vibrations, solder surface tension assembly, etc. have been used to get the mirror out of the plane of the wafer. Multi User MEMS Processes (MUMPs) process actuators were used to assemble the mirrors in these kinds of CCRs. MUMPs process is a three layer polysilicon micromachining process. It is a commercial program which provides the user micromachined fabricated devices using polysilicon as the structural material. The mirrors fabricated using MUMPs process suffer from misalignments of the mirror surfaces and also the mirror faces were not flat.
Accordingly, there remains a need in the art for an improved MEMS corner cube retroreflector and method of fabricating corner cube retroreflectors. The improved method should provide CCRs having high packing densities and optimal acceptance angles, while also maximizing the use of the chip area and reducing the operational power requirements of a cantilever.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified need could be fulfilled.